feat: defer statement support in both compilers

- Add  statement for function-level deferred execution
- Deferred expressions run in LIFO order on function exit (return or implicit)
- Bootstrap: desugar defers in hir_lower.nim (inject before return + end of func)
- Selfhost: emit defers in c_backend.bux via CEmitter defer stack
- Both: add tkDefer token, skDefer AST node, hDefer HIR node
- Parser, lexer, sema, token files updated in both bootstrap/ and src/

bootstrap/sema.nim

@@ -1335,6 +1335,9 @@ proc checkStmt(sema: var Sema, stmt: Stmt, scope: Scope): Type =
     elif not exprType.isUnknown and exprType.kind != tkStr:
       sema.emitError(stmt.loc, "#emit requires a string expression")
     return makeVoid()
+  of skDefer:
+    discard sema.checkExpr(stmt.stmtDeferBody, scope)
+    return makeVoid()
   of skDecl:
     # Local declaration inside block
     case stmt.stmtDecl.kind